The use of radial slots, traversed in one direction, for diaphragm glands with smooth slots, step slots, or labyrinth slots is generally known. All slots of this type are subject to the same circumstances. A pressure difference in the fluid at the end of the slot causes an axial flow through the slot, which depends on the slot width. Because the first structural part is rotating, a rotational flow is superposed on the axial flow through the slot. Since the slot, which has fluid flowing through it acts as a throttle slot, in which the pressure of the fluid is reduced, the slot is subject to powerful hydraulic influences. These influences make the slot a source of instabilities for the shaft, and especially for vibrations in the radial direction, which impair the quiet running of the shaft. Therefore, to prevent the troublesome instabilities, especially vibrations, bearings are typically placed as close as possible to the slot.
A radial slot, traversed in the axial direction, is also used in connection with axial-thrust balancing devices of turbo-machines, to control the axial thrust. These balancing devices can consist of a dummy piston guided in a bush (see Europump, Technical Vocabulary, Part 2.1, Components of Centrifugal Pumps, FIGS. 11c, 12). Balancing devices consisting of a combination of a dummy piston and a relief disk are also known (see Europump, Part 2.1, FIG. 29b). A disadvantage here is that the part of the balancing device, which is connected to the shaft, increases the mass of the shaft, thus further impairing the quiet running of the balancing device.
To eliminate this disadvantage, it is known from the EP-B 0 224 764 that the bush of a balancing device can be equipped with channels. The flow conducted through the channels to the slot divides into two flow directions: into a back-flow into the space on the rotor side, as a reverse current, and into the actual load-relief flow from the channels to the low-pressure side. The reverse current is designed to achieve a load-relief flow that is free of preliminary rotation. Just by reducing the rotation in the slot, this reduces the tendency of the pump rotor to proper vibrations in the limit-load region. However, as a result of this design, the balancing device becomes even longer and its mass becomes even greater than that of known balancing devices.